The present invention relates generally to magnetic resonance imaging (“MRI”) systems and methods and, more particularly, the invention relates to systems and methods for performing contrast-enhanced magnetic resonance angiography (CE-MRA) of a subject using high-spatial resolution, fluoroscopic images imbedded within the CE-MRA study as a trigger for coordinating the CE-MRA study with delivery of the contrast to the periphery.
Magnetic resonance imaging (“MRI”) uses the nuclear magnetic resonance (“NMR”) phenomenon to produce images. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in the x-y plane and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped,” into the x-y plane to produce a net transverse magnetic moment Mxy. A signal is emitted by the excited nuclei or “spins,” after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The measurement cycle used to acquire each MR signal is performed under the direction of a pulse sequence produced by a pulse sequencer. Clinically available MRI systems store a library of such pulse sequences that can be prescribed to meet the needs of many different clinical applications. Research MRI systems include a library of clinically-proven pulse sequences and they also enable the development of new pulse sequences.
The MR signals acquired with an MRI system are signal samples of the subject of the examination in Fourier space, or what is often referred to in the art as “k-space.” Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. Most pulse sequences sample k-space in a raster scan-like pattern sometimes referred to as a “spin-warp,” a “Fourier,” a “rectilinear,” or a “Cartesian” scan. The spin-warp scan technique employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of MR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (“2DFT”), for example, spatial information is encoded in one direction by applying a phase encoding gradient, Gy, along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient, Gx, in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse, Gy, is incremented, ΔGy, in the sequence of measurement cycles, or “views” that are acquired during the scan to produce a set of k-space MR data from which an entire image can be reconstructed.
Magnetic resonance angiography (MRA) uses the magnetic resonance phenomenon to produce images of the human vasculature. To enhance the diagnostic capability of MRA a contrast agent such as gadolinium can be injected into the patient prior to the MRA scan. Typically a moderate amount (10-30 ml) of a gadolinium-based contrast agent is injected into an arm vein. The contrast material then mixes with the systemic blood in the heart and pulmonary vasculature and passes from the left heart into the arterial circulation. The presence of contrast material in the blood causes the net T1 relaxation time to be altered from its unenhanced value, for example, of about 1000 msec to values in the range, for example, of 50 to 100 msec. MR acquisition methods can exploit this change in T1, causing the enhanced blood within the vasculature to be significantly brighter compared to other structures within the imaging FOV.
There are a wide variety of technical challenges to performing CE-MRA to yield the desired information for a particular setting. As described in U.S. Pat. No. 5,417,213 the trick with this CE-MRA method is to acquire the central k-space views at the moment the bolus of contrast agent is flowing through the vasculature of interest. Collection of the central lines of k-space during peak arterial enhancement is key to the success of a CE-MRA exam. If the central lines of k-space are acquired prior to the arrival of contrast, severe image artifacts can limit the diagnostic information in the image. Alternatively, arterial images acquired after the passage of the peak arterial contrast are sometimes obscured by the enhancement of veins. In many anatomic regions, such as the carotid or renal arteries, the separation between arterial and venous enhancement can be as short as 6 seconds. However, this timing constraint is in opposition with the need to obtain a high spatial resolution image, for example, a three-dimensional (3D) image with adequate spatial resolution. To do so, it is necessary to have a sufficiently long acquisition time, generally in the range of ten seconds or longer, in order to collect enough information to yield the desired spatial resolution
The short separation time between arterial and venous enhancement dictates the use of acquisition sequences of either low spatial resolution or very short repetition times (TR). Short TR acquisition sequences severely limit the signal-to-noise ratio (SNR) of the acquired images relative to those exams in which longer TRs are possible. The rapid acquisitions required by first-pass CE-MRA methods thus impose an upper limit on either spatial or temporal resolution.
As a result, depending on the technique used and the trade-offs that may be tolerated in a given clinical setting, it may be possible to utilize one of a variety of different strategies that have been developed to shorten the scan time. For example, one such strategy is referred to generally as “parallel MRI” (“pMRI”). Parallel MRI techniques use spatial information from arrays of radio frequency (“RF”) receiver coils to substitute for the spatial encoding that would otherwise have to be obtained in a sequential fashion using RF pulses and magnetic field gradients, such as phase and frequency encoding gradients. Each of the spatially independent receiver coils of the array carries certain spatial information and has a different spatial sensitivity profile. This information is utilized in order to achieve a complete spatial encoding of the received MR signals, for example, by combining the simultaneously acquired data received from each of the separate coils.
Parallel MRI techniques allow an undersampling of k-space by reducing the number of acquired phase-encoded k-space sampling lines, while keeping the maximal extent covered in k-space fixed. The combination of the separate MR signals produced by the separate receiver coils enables a reduction of the acquisition time required for an image, in comparison to a conventional k-space data acquisition, by a factor generally bounded by the number of the receiver coils. Thus, the use of multiple receiver coils acts to multiply imaging speed, without increasing gradient switching rates or RF power.
Two categories of such parallel imaging techniques that have been developed and applied to in vivo imaging are so-called “image space methods” and “k-space methods.” An exemplary image space method is known in the art as sensitivity encoding (“SENSE”), while an exemplary k-space method is known in the art as simultaneous acquisition of spatial harmonics (“SMASH”). With SENSE, the undersampled k-space data is first Fourier transformed to produce an aliased image from each coil, and then the aliased image signals are unfolded by a linear transformation of the superimposed pixel values. With SMASH, the omitted k-space lines are synthesized or reconstructed prior to Fourier transformation, by constructing a weighted combination of neighboring k-space lines acquired by the different receiver coils. SMASH requires that the spatial sensitivity of the coils be determined, and one way to do so is by “autocalibration” that entails the use of variable density k-space sampling. A more recent advance to SMASH techniques using autocalibration is a technique known as generalized autocalibrating partially parallel acquisitions (“GRAPPA”), as described, for example, in U.S. Pat. No. 6,841,998. With GRAPPA, k-space lines near the center of k-space are sampled at the Nyquist frequency, in comparison to the undersampling employed in the peripheral regions of k-space. These center k-space lines are referred to as the so-called autocalibration signal (“ACS”) lines, which are used to determine the weighting factors that are utilized to synthesize, or reconstruct, the missing k-space lines.
When applied to CE-MRA acquisitions, short repetition time (TR) gradient echo sequences allow rapid collection of MRI data, and this can be accelerated with undersampling techniques such as SENSE and GRAPPA. Synchronizing the acquisition to the contrast arrival can be done using a test bolus or fluoroscopic triggering, such as described by Wilman A H, Riederer S J, King B F, Debbins J P, Rossman P J, Ehman R L. Fluoroscopically-triggered contrast-enhanced three-dimensional MR angiography with elliptical centric view order: application to the renal arteries. Radiology 1997; 205:137-146. An extension of the acquisition duration well into the venous phase, but with negligible venous signal, can be done using various centric phase encoding view orders, such as described by Wilman A H, Riederer S J. Performance of an elliptical centric view order for signal enhancement and motion artifact suppression in breathhold three dimensional gradient echo imaging. Magn Reson Med 1997; 38:793-802.
Imaging of the peripheral vasculature, defined roughly as those vessels contained within the region extending from the pelvis to the feet, poses additional technical challenges. First, the volume to be imaged is longer than the active region of the magnet system of the MRI system and, thus, the field of view (FOV) available to the clinician. To address this initial challenge, CE-MRA of the peripheral vasculature is generally done by breaking the extended superior-inferior (S/I) region into a series of three to five individual “stations,” each of which can be imaged individually within the FOV of the MRI system. That is, MRA data is acquired from a desired, large field of view that extends beyond the FOV of the MRI system by moving the patient table to a plurality of different locations during the scan and acquiring an image at each station. The movement of the table should ideally be timed to follow the contrast bolus through the vasculature so that peak arterial contrast is imaged at each station. One method for matching image acquisition to peak arterial contrast, but limited to the first station, is fluoroscopic triggering. When contrast arrival is observed fluoroscopically within the first station, the radiologist or technologist can trigger the 10-25 sec long 3D acquisition of that first station. However, at the time of triggering the real-time presentation of contrast advance ceases, and the observer has no knowledge about the subsequent speed of advance of the contrast bolus to subsequent stations. Because this speed of advance is a highly patient-specific parameter, this approach is prone to error. The operator is in effect “flying blindly.”
A second issue is that it is desirable to acquire data for an adequately long time at a given station to insure adequate image quality and sufficient spatial resolution but still an adequately short time to allow the scanner to advance to the next station prior to any venous enhancement there. These are competing demands. Sometimes this is addressed by applying an inflatable compression cuff around the patient's thighs to inhibit or delay blood passage from the arterial to the venous beds. However, use of venous compression cuffs adds extra time to the CE-MRA exam, and arguably images a non-normal physiological situation.
A third issue, as alluded to above, is that the transit time of the contrast bolus along the peripheral vasculature is highly variable from patient to patient. An attempted solution to this problem is to first administer a small, 2 ml test bolus and observe the arrival time at one or more locations along the peripheral vasculature and use this information to guide the time spent at each station. Various means have been developed to provide accurate timing including: use of a small test injection of contrast as described by J. K. Kim, R. I. Farb, and G. A. Wright, Test Bolus Examination in the Carotid Artery at Dynamic Gadolinium-enhanced MR Angiography, Radiology, 1998, 206:283-289. However, use of a test bolus also adds to the overall exam time and provides undesirable background signal to the subsequent high contrast dose exam.
Despite the advent of these and other techniques for addressing the competing clinical challenges of CE-MRA acquisitions and generally improving CE-MRA, none of the above solutions is completely effective in controlling or balancing the competing clinical challenges of CE-MRA acquisitions, particularly when the clinical need dictates a CE-MRA study of the periphery of a subject.
It would therefore be desirable to provide a system and method for performing angiographic studies of the periphery of a subject that provides high spatial resolution imaging of the arterial vasculature with sufficient venous suppression, but without the need of extraneous processes that extend the overall procedure, such as the use of a preliminary test bolus or compression cuffs. Furthermore, such systems and methods should be patient-specific to yield desirable, patient-specific clinical information.